Editing IPCC:AR6/SROCC/Chapter-4 (section)

==== 4.4.2.2 Hard and Sediment-Based Protection ====

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<span id="observed-hard-and-sediment-based-protection-across-geographies"></span>
===== 4.4.2.2.1 Observed hard and sediment-based protection across geographies =====

Coastal protection through hard measures is widespread around the world, although it is difficult to provide estimates on how many people benefit from them. Currently, at least 20 million people living below normal high tides are protected by hard structures (and drainage) in countries such as Belgium, Canada, China, Germany, Italy, Japan, the Netherlands, Poland, Thailand, the UK, and the USA (Nicholls, 2010). Many more people living above high tides are also protected against ESL by hard structures in major cities around the world. There is a concentration of these measures in northwest Europe and East Asia, although extensive defences are also found in and around many coastal cities and deltas. For example, large scale coastal protection exists in Vancouver (Canada), Alexandria (Egypt) and Keta (Ghana; Nairn et al., 1999 <sup>[[#fn:r1544|1544]]</sup> ) and 6000 km of polder dikes in coastal Bangladesh. Gittman et al. (2015) estimate that 14% of the total US coastline has been armoured, with New Orleans being an example of an area below sea level dependent on extensive engineered protection (Kates et al., 2006 <sup>[[#fn:r1545|1545]]</sup> ; Rosenzweig and Solecki, 2014 <sup>[[#fn:r1546|1546]]</sup> ; Cooper et al., 2016 <sup>[[#fn:r1547|1547]]</sup> ). Defences built and raised for tsunami protection, such as post-2011 in Japan (Raby et al., 2015 <sup>[[#fn:r1548|1548]]</sup> ), also provide protection against SLR.

The application of sediment-based protection measures also has a long history, offering multiple benefits in terms of enhancing safety, recreation and natural systems (JSCE, 2000 <sup>[[#fn:r1549|1549]]</sup> ; Dean, 2002 <sup>[[#fn:r1550|1550]]</sup> ; Hanson et al., 2002 <sup>[[#fn:r1551|1551]]</sup> ; Cooke et al., 2012 <sup>[[#fn:r1552|1552]]</sup> ). About 24% of the world’s sandy beaches are currently eroding by rates faster than 0.5 m yr–1 (Luijendijk et al., 2018 <sup>[[#fn:r1553|1553]]</sup> ). In the USA, Europe and Australia, these responses are often driven by the recreational value of beaches and the high economic benefits associated with beach tourism. More recently, sediment-based measures are implemented as effective and yet flexible measures to address SLR (Kabat et al., 2009 <sup>[[#fn:r1554|1554]]</sup> ) and experiments are being conducted with innovative decadal scale application of sediments such as the sand engine in the Netherlands (Stive et al., 2013 <sup>[[#fn:r1555|1555]]</sup> ).

There is high confidence that most major upgrades in defences happen after coastal disasters (Box 4.1). Dikes were raised and reienforced after the devastating coastal flood of 1953 in the Netherlands and the UK, and in 1962 in Germany. In New Orleans, investments in the order of 15 billion USD, including a major storm surge barrier, followed Hurricane Katrina in 2005 (Fischetti, 2015 <sup>[[#fn:r1556|1556]]</sup> ), and in New York the Federal Government made available 16 billion USD for disaster recovery and adaptation after Superstorm Sandy in 2012 (NYC, 2015). Examples in which SLR has been considered proactively in the planning process include SLR safety margins in, for example, the UK, Germany and France, upgrading defences according to cost-benefit analysis in the Netherlands, and SLR guidance in the USA (USACE, 2011 <sup>[[#fn:r1557|1557]]</sup> ).

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<span id="projected-hard-and-sediment-based-protection"></span>
===== 4.4.2.2.2 Projected hard and sediment-based protection =====

There is ''high confidence'' that hard coastal protection will continue to be a widespread response to SLR in densely populated and urban areas during the 21st century, because this response is widely practised (Section 4.4.2.2.2), effective in reducing current (Section 4.4.2.2.2) and future flood risk (Section 4.3.3.2) and highly cost efficient in urban and densely populated areas (Section 4.4.2.7). There is, however, ''low agreement'' on the level of hard coastal protections to expect, with projections being based on different assumptions. A model assuming that coastal societies upgrade hard protection following scenario-based cost-benefit analysis finds that 22% of the global coastline will be protected under various SSPs and 1 m of 21st century global mean SLR (Nicholls et al., 2019 <sup>[[#fn:r1558|1558]]</sup> ). Another model assuming that only areas for which benefit-cost ratios are above 1 under SLR scenarios up to 2 m, all SSPs and discount rates up to 6%, finds that this would lead to protecting 13% of the global coastline (Lincke and Hinkel, 2018 <sup>[[#fn:r1559|1559]]</sup> ; Figure 4.14).

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<span id="cost-of-hard-and-sediment-based-protection"></span>
===== 4.4.2.2.3 Cost of hard and sediment-based protection =====

There is ''medium evidence'' and ''medium agreement'' on the costs of hard protection. Data on the costs of hard defences is only available for few countries and unit costs estimated from this data vary substantially depending on building/fill material used, labour cost, urban versus rural settings, hydraulic loads, etc. (Jonkman et al., 2013 <sup>[[#fn:r1587|1587]]</sup> ; Lenk et al., 2017 <sup>[[#fn:r1588|1588]]</sup> ; Aerts, 2018 <sup>[[#fn:r1589|1589]]</sup> ; Nicholls et al., 2019 <sup>[[#fn:r1590|1590]]</sup> ). In general, there has been limited systematic data collection across sites, although useful national guidance does exist in some cases (Environment Agency, 2015 <sup>[[#fn:r1591|1591]]</sup> ). Defences depend on good maintenance to remain effective. For some types of infrastructure such as surge barriers, maintenance costs are poorly described and hence more uncertain (Nicholls et al., 2007 <sup>[[#fn:r1592|1592]]</sup> ). Protection-based adaptation to saltwater intrusion is more complex than adaptation to flooding and erosion, and there is less experience to draw upon.

Based on these unit cost estimates, and different assumptions on future protection, global annual protection costs have been estimated to be 12–71 billion USD considering coastal dikes only (Hinkel et al., 2014 <sup>[[#fn:r1593|1593]]</sup> ) and about 40–170 billion USD yr -1 considering coastal dikes, river dikes and storm surge barriers, under RCP2.6, and about 25–200 billion USD yr -1 considering coastal dikes only (Tamura et al. 2019 <sup>[[#fn:r1594|1594]]</sup> ) under RCP8.5. If protection is widely practised through the 21st century, the bulk of the costs will be maintenance rather than capital costs (Nicholls et al., 2019 <sup>[[#fn:r1595|1595]]</sup> ).

<span id="table-4.7"></span>
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'''Table 4.7'''

'''Table 4.7:''' Capital and maintenance costs of hard protection measures.

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{| class="wikitable"
|-
| Measure
| Capital cost (in million USD unless stated otherwise)
| Annual Maintenance Cost (% of capital cost)
|-
| Sea Wall
| 0.4–27.5 km -1 length and metre height (Linham et al., 2010)
| 1–2% per annum (Jonkman et al., 2013)
|-
| Sea Dike
| 0.9–69.9 km -1 length and metre height (Jonkman et al., 2013; Nicholls et al., 2019; Tamura et al., 2019)
| 1–2% per annum (Jonkman et al., 2013)
|-
| Breakwater
| 2.5–10.0 km -1 length (Narayan et al., 2016)
| 1% per annum (Jonkman et al., 2013)
|-
| Storm Surge Barrier
| 0.9–2.7 (Jonkman et al., 2013) or 2.2 (Mooyaart and Jonkman, 2017) million EUR per metre width
| 1% per annum (Mooyaart and Jonkman, 2017) or 5–10% per annum (Nicholls et al., 2007)
|-
| Saltwater
Intrusion Barriers

| Limited knowledge
|}
<!-- END TABLE -->

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Sediment-based measures are generally costed as the unit cost of sand (or gravel) delivery multiplied by the volumetric demand. Unit costs range from 3–21 USD m <sup>–</sup> ³ sand , with some high outlier costs in, for example, the UK, South Africa and New Zealand (Linham et al., 2010 <sup>[[#fn:r1596|1596]]</sup> ; Aerts, 2018 <sup>[[#fn:r1597|1597]]</sup> ). Costs are small where sources of sand are plentiful and close to the sites of demand. Costs are further reduced by shoreface nourishment approaches. The Netherlands maintains its entire open coast with large-scale shore nourishment (Mulder et al., 2011 <sup>[[#fn:r1598|1598]]</sup> ) and the innovative sand engine has been implemented as a full-scale decadal experiment (Stive et al., 2013 <sup>[[#fn:r1599|1599]]</sup> ). The capital costs for dunes are similar to beach nourishment, although placement and planting vegetation may raise costs. Maintenance costs vary from almost nothing to several million USD km <sup>–1</sup> , although costs are usually at the lower end of this range (Environment Agency, 2015 <sup>[[#fn:r1600|1600]]</sup> ).

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<span id="effectiveness-of-hard-and-sediment-based-protection"></span>
===== 4.4.2.2.4 Effectiveness of hard and sediment-based protection =====

There is ''high confidence'' that well designed and maintained hard and sediment-based protection is very effective in reducing risk to the impacts of SLR and ESL (Horikawa, 1978 <sup>[[#fn:r1572|1572]]</sup> ; USACE, 2002 <sup>[[#fn:r1673|1673]]</sup> ; CIRIA, 2007 <sup>[[#fn:r1674|1674]]</sup> ). This includes situations in which coastal megacities in river deltas have experienced, and adapted to, relative SLR of several metres caused by land subsidence during the 20th century (Kaneko and Toyota, 2011 <sup>[[#fn:r1675|1675]]</sup> ; Esteban et al., 2019 <sup>[[#fn:r1676|1676]]</sup> ; Box 4.1). In principle, there are no technological limits to protect the coast during the 21st century even under high-end SLR of 2 m (Section 4.3.3.2), but technological challenges can make protection very expensive and hence unaffordable in some areas (Hinkel et al., 2018 <sup>[[#fn:r1677|1677]]</sup> ). Examples include southeast Florida, because protected areas can be flooded by rising groundwater through underlying porous limestone (Bloetscher et al., 2011 <sup>[[#fn:r1678|1678]]</sup> ). Gradually rising water tables behind defences is also an issue, which can be managed by increasing pumping and drainage (Aerts, 2018 <sup>[[#fn:r1679|1679]]</sup> ). Maintaining this effectiveness over time requires regular monitoring and maintenance, accounting for changing conditions such as SLR and widespread erosional trends in front of the defences. There will always be residual risks, which can be reduced, but never eliminated, by engineering protection infrastructure to very high standards, such as so-called ‘unbreakable dikes’ (de Bruijn et al., 2013).

It is difficult to assess at what point in time and for which amount of SLR technical limits for coastal protection will be reached. Parts of Tokyo have been protected against five metres of relative SLR during the 21st century (Kaneko and Toyota, 2011 <sup>[[#fn:r1680|1680]]</sup> ) and it has been argued that it is possible to preserve territorial integrity of the Netherlands even under 5 m SLR, using current engineering technology (Aerts et al., 2008 <sup>[[#fn:r1681|1681]]</sup> ; Olsthoorn et al., 2008 <sup>[[#fn:r1682|1682]]</sup> ). This suggests that under RCP2.6, technical limits to adaptation will be rare even under longer-term SLR. Protecting against high-end SLR will be increasingly technically challenging as we move beyond the 21st century. This is not only due to the absolute amount of SLR, but also due to the very high rates of annual SLR (e.g., 10–20 mm yr –1 ''likely'' range under RCP8.5 in 2100), which challenge the planning and implementation of hard protection because major protection infrastructure requires decades to plan and implement (Gilbert et al., 1984 <sup>[[#fn:r1683|1683]]</sup> ; Burcharth et al., 2014 <sup>[[#fn:r1684|1684]]</sup> ). In summary, the higher and faster SLR, the more challenging coastal protection will be, but quantifying this is difficult. In any case, before technical limits are reached, economic and social limits will be reached because societies are neither economically able nor socially willing to invest in coastal protection (Sections 4.4.2.2 and 4.3.3.2; Hinkel et al., 2018; Esteban et al., 2019).

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<span id="co-benefits-and-drawbacks-of-hard-and-sediment-based-protection"></span>
===== 4.4.2.2.5 Co-benefits and drawbacks of hard and sediment-based protection =====

When space is limited (e.g., in an urban setting), co-benefits can be generated through multi-functional hard flood defences, which combine flood protection with other urban functions, such as car parks, buildings, roads or recreational spaces into one multifunctional structure (Stalenberg, 2013 <sup>[[#fn:r1601|1601]]</sup> ; van Loon-Steensma and Vellinga, 2014 <sup>[[#fn:r162|162]]</sup> ). An important co-benefit of sediment-based protection, such as beach nourishment and dune management, is that it preserves beach and associated environments, as well as tourism (Everard et al., 2010 <sup>[[#fn:r1603|1603]]</sup> ; Hinkel et al., 2013a <sup>[[#fn:r1604|1604]]</sup> ; Stive et al., 2013 <sup>[[#fn:r1605|1605]]</sup> ).

Drawbacks of hard protection include the alteration of hydrodynamic and morphodynamic patterns, which in turn may export flooding and erosion problems downdrift (Masselink and Gehrels, 2015 <sup>[[#fn:r1606|1606]]</sup> ; Nicholls et al., 2015 <sup>[[#fn:r1607|1607]]</sup> ). For example, protection of existing shoreline in estuaries and tidal creeks may increase tidal amplification in the upper parts (Lee et al., 2017 <sup>[[#fn:r1608|1608]]</sup> ). Hard protection also hinders or prohibits the onshore migration of geomorphic features and ecosystems (called coastal squeeze; Pontee, 2013 <sup>[[#fn:r1609|1609]]</sup> ; Gittman et al., 2016 <sup>[[#fn:r1610|1610]]</sup> ), leading to both a loss of habitat as well as of the protection function of ecosystems (see Sections 4.3.2.4 and 4.4.2.2). Another drawback of raising hard structures, also emphasised in AR5, is the risk of lock-in to a development pathway in which development intensifies behind higher and higher defences, with escalating severe consequences in the event of protection failure (Wong et al., 2014 <sup>[[#fn:r1611|1611]]</sup> ; Welch et al., 2017 <sup>[[#fn:r1612|1612]]</sup> ), as experienced in Hurricane Katrina impacted New Orleans (Burby, 2006 <sup>[[#fn:r1613|1613]]</sup> ; Freudenburg et al., 2009 <sup>[[#fn:r1614|1614]]</sup> ). This lock-in results from protection attracting further economic development in the flood zone within defenses, which then leads to further raising defences with SLR, and the growing value of exposed assets.

Seabed dredging of sand and gravel can have negative impacts on marine ecosystems such as seagrass meadows and corals (Erftemeijer and Lewis III, 2006 <sup>[[#fn:r1615|1615]]</sup> ; Erftemeijer et al., 2012 <sup>[[#fn:r1616|1616]]</sup> ). Nourishment practices on sandy beaches have also been shown to have drawbacks for local ecosystems if local habitat factors are not taken into consideration when planning and implementing nourishment and maintenance (Speybroeck et al., 2006 <sup>[[#fn:r1617|1617]]</sup> ). A further emerging issue is beach material scarcity mainly driven by demand of sand and gravel for construction, but also for beach and shore nourishment (Peduzzi, 2014 <sup>[[#fn:r1618|1618]]</sup> ; Torres et al., 2017 <sup>[[#fn:r1619|1619]]</sup> ), which makes sourcing the increasing volumes of beach materials required to sustain beaches in the face of SLR more expensive and challenging (Roelvink, 2015 <sup>[[#fn:r1620|1620]]</sup> ).

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<span id="governance-of-hard-and-sediment-based-protection"></span>
===== 4.4.2.2.6 Governance of hard and sediment-based protection =====

Reviews and comparative case studies confirm findings of AR5 that governance challenges are amongst the most common hindrance to implementing coastal measures (Ekstrom and Moser, 2014 <sup>[[#fn:r1621|1621]]</sup> ; Hinkel et al., 2018 <sup>[[#fn:r1622|1622]]</sup> ). One main issue to resolve is conflicting stakeholder interests. This includes conflicts between those favouring protection and those being negatively affected by adaptation measures. In Catalonia, for example, the tourism sector welcomes beach nourishment because it provides direct benefits, whereas those dependent upon natural resources (e.g., fishermen) are increasingly in opposition because they fear that sand mining destroys coastal habitat and livelihood prospects (González-Correa et al., 2008 <sup>[[#fn:r1623|1623]]</sup> ).

There is also conflict related to the distribution of public money between communities receiving public support for adaptation and non-coastal communities who pay for this support through taxes (Elrick-Barr et al., 2015 <sup>[[#fn:r1624|1624]]</sup> ). Generally, access to financial resources for adaptation, including from public sources, development and climate finance or capital markets, frequently constrain adaptation (Ekstrom and Moser, 2014 <sup>[[#fn:r1625|1625]]</sup> ; Hinkel et al., 2018 <sup>[[#fn:r1626|1626]]</sup> ). For example, homeowners are often not willing to pay taxes or levies for public protection or sediment-base measures even if they directly benefit, as found, for example in communities on the US east coast where beach nourishment is used to maintain recreational and tourism amenities (Mullin et al., 2019 <sup>[[#fn:r1627|1627]]</sup> ). In many parts of the world, coastal adaptation governance is further complicated by existing conflicts over resources. For example, illegal coastal sand mining is currently a major driver of coastal erosion in many parts of the developing world (Peduzzi, 2014 <sup>[[#fn:r1628|1628]]</sup> ). Examples of this can be found in Ghana (Addo, 2015 <sup>[[#fn:r1629|1629]]</sup> ) and the Comoros (Betzold and Mohamed, 2017 <sup>[[#fn:r1630|1630]]</sup> ).

An associated governance challenge is ensuring the effective maintenance of coastal protection. Ineffective maintenance has contributed to many coastal disasters in the past, such as in New Orleans (Andersen, 2007 <sup>[[#fn:r1631|1631]]</sup> ). AR5 highlighted that effective maintenance is challenging in a small island context due to a lack of adequate funds, policies and technical skills (Nurse et al., 2014 <sup>[[#fn:r1632|1632]]</sup> ). In some countries in which coastal defence systems have a long history, effective governance arrangements for maintenance, such as the Water Boards in the Netherlands, have emerged. In Bangladesh, where Dutch-like polders were introduced in the 1960s, maintenance has been a challenge due to shifts in multi-level governance structures associated with independence, national policy priorities and donor involvement (Dewan et al., 2015 <sup>[[#fn:r1633|1633]]</sup> ).

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<span id="economics-of-coastal-adaptation"></span>
===== 4.4.2.2.7 Economics of coastal adaptation =====

At global scales, new economic assessments of responses have mostly focused on the direct costs of hard protection and the benefits of reducing coastal extreme event flood risks. These studies confirm AR5 findings that the benefits of reducing coastal flood risk through hard protection exceed the costs of protection, on a global average, and for cities and densely populated areas, during the 21st century even under high-end SLR ( ''medium evidence, high agreement'' ; Hallegatte et al., 2013 <sup>[[#fn:r1634|1634]]</sup> ; Wong et al., 2014 <sup>[[#fn:r1635|1635]]</sup> ; Diaz, 2016 <sup>[[#fn:r1636|1636]]</sup> ; Lincke and Hinkel, 2018 <sup>[[#fn:r1637|1637]]</sup> ). For example, Lincke and Hinkel (2018) find that, during the 21st century, it is economically efficient to protect 13% of the global coastline, which corresponds to 90% of global floodplain population, under SLR scenarios from 0.3–2.0 m, five SSPs and discount rates up to 6% (Figure 4.14). While the above two studies have not considered the effects of hard protection in reducing the area of coastal wetlands, it is expected that coastal hard protection in densely populated areas and conserving wetlands in sparsely populated areas can go hand in hand. Protecting less than 42% of the global coastline would leave coastal wetlands sufficient accommodation space to even grow in areas under rising sea levels during the 21st century (Schuerch et al., 2018 <sup>[[#fn:r1639|1639]]</sup> ). Diaz (2016), who includes the cost of wetland loss, using a simpler wetland model, finds that both protection and retreat reduce the global net present costs of SLR by a factor of seven as compared to no adaptation (applying a discount rate of 4%) under 21st century SLR of 0.3–1.3 m and SSP2. There is no global study that has considered social costs and benefits of responses (e.g., health, beach amenity, etc.) or looked at the economics of accommodate, retreat and advance responses.

<span id="figure-4.14"></span>
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'''Figure 4.14'''

<span id="figure-4.14-economic-robustness-of-coastal-protection-under-sea-level-rise-slr-scenarios-from-0.32.0-m-the-five-shared-socioeconomic-pathways-ssps-and-discount-rates-of-up-to-6.-coastlines-are-coloured-according-to-the-percentage-of-scenarios-under-which-the-benefit-cost-ratio-of-protection-reduced-flood-risk-divided-by-the-cost-of-protection"></span>
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'''Figure 4.14 | Economic robustness of coastal protection under sea level rise (SLR) scenarios from 0.3–2.0 m, the five Shared Socioeconomic Pathways (SSPs) and discount rates of up to 6%. Coastlines are coloured according to the percentage of scenarios under which the benefit-cost ratio of protection (reduced flood risk divided by the cost of protection) […]'''
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[[File:b867d55a5f3933dfe77ecb75a01cc583 IPCC-SROCC-CH_4_14-3000x1772.jpg]]

Figure 4.14 | Economic robustness of coastal protection under sea level rise (SLR) scenarios from 0.3–2.0 m, the five Shared Socioeconomic Pathways (SSPs) and discount rates of up to 6%. Coastlines are coloured according to the percentage of scenarios under which the benefit-cost ratio of protection (reduced flood risk divided by the cost of protection) are above 1. Source: Lincke and Hinkel (2018).

At local scales, a large number of economic assessments of response options are available but mostly in the grey literature and again with a focus on hard and sediment-based protection. Similar to the global studies, hard protection is generally found to be economically efficient for urban and densely populated areas such as New York, USA (Aerts et al., 2014 <sup>[[#fn:r1640|1640]]</sup> ) and Ho Chi Minh City, Vietnam (Scussolini et al., 2017 <sup>[[#fn:r1641|1641]]</sup> ). Both global and local studies show that sediment-based protection, such as beach nourishment is economically efficient in areas of intensive tourism development due to the large revenues generated within this sector (Rigall-I-Torrent et al., 2011 <sup>[[#fn:r1642|1642]]</sup> ; Hinkel et al., 2013a <sup>[[#fn:r1643|1643]]</sup> ).

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<div id="section-4-4-2-3ecosystem-based-adaptation"></div>

<span id="ecosystem-based-adaptation"></span>